1. Field of the Invention
The invention relates, in general, to a method and apparatus for controlling the transport and the positioning of sheets in printers or the like, comprising a light source as well as photoelectric elements located at a distance from, and opposite such light source.
2. Description of the Prior Art
A typical photoelectric measuring unit for use in sheet-finishing and sheet-processing machines is shown in German OS 2 202 087. In that unit, a plurality of photoelectric receivers are arranged in the area of the reference line. Each individual photoelectric receiver or each group of such receivers is associated with a light source. The distance of the individual receivers results in a measuring accuracy in the range of tenths of a millimeter. The sheet to be transported is guided in the area between the light sources and the photoelectric receivers. Depending on how far the sheet has moved into the ray path between light source and receiver, a corresponding number of the receivers is shielded. Using a suitable evaluation circuit, the position of the sheet can be determined within the range of the measuring accuracy attainable in the device concerned. This unit only allows the position of an incoming sheet to be monitored while not allowing the positioning or the transport of the sheets to be controlled.
Further, European Patent No. EP 0 152 873 discloses a device for controlling sheet transport in a printer. For this purpose, slotted disks are used which are mounted on the driving shaft of the sheet transport motor. Each of the slotted disks is associated with a photoelectric element which is used for detecting the individual slots in the disk. The electric signals generated by the photoelectric elements serve to control the stepping motor of the sheet transport roller. By means of these signals a fine adjustment of the stepping motor is possible. However, this device is unable to compensate for slip-related changes in the position of the sheet during sheet transport.